The Airborne Transmission of Viruses Causes Tight Transmission Bottlenecks

The transmission bottleneck describes the number of viral particles that found an infection in a new host. Previous studies have used genome sequence data to suggest that transmission bottlenecks for influenza and SARS-CoV-2 involve few viral particles, but the general principles underlying these bottlenecks are not fully understood. Here we show that, across a broad range of circumstances, tight transmission bottlenecks arise as a consequence of the physical process underlying airborne viral transmission. We use a mathematical model to describe the process of infectious particles being emitted by an infected individual and inhaled by others nearby. The extent to which exposure to particles translates into infection is determined by an effective viral load, which is calculated as a function of the epidemiological parameter R. Across multiple scenarios, including those present at a superspreading event, our model suggests that the great majority of transmission bottlenecks involve few viral particles, with a high proportion of infections being caused by a single viral particle. Our results provide a physical explanation for previous inferences of bottleneck size and predict that tight transmission bottlenecks prevail more generally in respiratory virus transmission. ### Competing Interest Statement The authors have declared no competing interest.